Answer:
C. [tex] = 36 \sqrt{7} {cm}^{2} [/tex]
Step-by-step explanation:
[tex]3 \sqrt{3} \times 4 \sqrt{21} {cm}^{2} [/tex]
[tex]12 \sqrt{63} {cm}^{2} = \sqrt{ {3}^{2} \times 7 } \\ = \sqrt{ {3}^{2} } \sqrt{7} = 3 \sqrt{7} [/tex]
[tex]12 \times 3 \sqrt{7} {cm}^{2} [/tex]
[tex]\boxed{\green{ = 36 \sqrt{7} {cm}^{2}}} [/tex]
